No abstract

Actinyl-actinyl interactions are particularly prevalent for the pentavalent neptunyl cation (Np(V)O 2 ) + where these interactions appear either as a T-or D-shape (diamond-shape). Tshaped interactions have been previously identified in high concentration Np(V) solutions containing simple anions (NO 3 À , ClO 4À , Cl À ) whereas D-shaped have only been isolated in the solid-state in the presence of carboxylate ligands. In this study, Density Functional Theory (DFT) calculations were paired with Raman spectroscopy to evaluate the formation of D-shaped interactions in the presence of aliphatic (R = H (formate), CH 3 (acetate), CH 2 CH 3 (propionate)) and aromatic (R = C 6 H 5 (benzoate), C 6 H 4 OH (4-hydroxybenzoate), C 5 H 4 N (isonicotinate)) carboxylate ligands. DFT studies indicate that the ΔG to form hydrated T-and D-shaped forms are not spontaneous but become so with the addition of the carboxylate ligands. Raman spectra of the Np(V) carboxylate solutions contained vibrational modes associated with the D-shaped interactions, but spectral changes observed over time indicate a dynamic system. Crystallization experiments from the Np(V) carboxylate systems confirmed the presence of D-shaped dimers for the aromatic carboxylates, suggesting that the choice of the anion in solution favors actinyl-actinyl interactions even at low concentrations (� 20 mM) of Np(V).